How does Nick catch Gatsby in a lie?

Gatsby told Nick he had inherited his money, but when they have Daisy to tea, he let's it slip that he "earned" it.

“Yes.” His eyes went over it, every arched door and square tower. “It took me just three years to earn the money that bought it.”

“I thought you inherited your money.”

“I did, old sport,” he said automatically, “but I lost most of it in the big panic — the panic of the war.”